High-resolution Imaging of Two Radio Quasars at the End of the Reionization Epoch

TL;DR

This study presents high-resolution VLBI imaging of two extremely distant quasars at the end of the reionization epoch, revealing their radio structures and spectral properties for the first time.

Contribution

First VLBI imaging of two high-redshift quasars, providing insights into their radio jet properties and spectral classifications.

Findings

VIKING J231818.35-311346.3 shows young radio jet features.

FIRST J233153.20+112952.11 exhibits characteristics of BL Lac objects.

Both sources are compact with flat-spectrum radio emission.

Abstract

There are approximately 250 quasars discovered at redshift $z\geq6$, of which only a handful were detected in radio bands, and even fewer were imaged with the highest resolution very long baseline interferometry (VLBI) technique. Here we report the results of our dual-frequency observations with the Very Long Baseline Array (VLBA) of two such recently discovered quasars, VIKING J231818.35$-$311346.3 at $z=6.44$ and FIRST J233153.20$+$112952.11 at $z=6.57$. Both extremely distant sources were imaged with VLBI for the first time. The radio properties of the former are consistent with those of quasars with young radio jets. The latter has an UV/optical spectrum characteristic of BL Lac objects, of which no others have been found beyond redshift 4 so far. Our VLBA observations revealed a flat-spectrum compact radio source.